High-gain,broadband traveling wave master



Dec. 23. 1969 TI 0. PAINE 3,486,123 ACTING ADMINISTRATOR OF THE NATIONAL AERONAUTICS AND SPACE ADMINESTRATION HIGH-GAIN, BROADBAND TRAVELING WAVE MASER 2 Sheets-Sheet 1 Filed Nov. 12, 1968 INVENTOR. ROBERT C. CLAUSS ATTO NEYS Dec. 23, 1969 T. o. PAINE 3,486,123

ACTING ADMINISTRATOR OF THE NATIONAL AERONAUTICS AND SPACE ADMiNlSTRATlON vI'IIGH-GrAIN, BROADBAND TRAVELING WAVE MASER Filed Nov. 12, 1968 2 Sheets-Sheet 2 INVENTOR. ROBERT C. CLAUSS ATTOFfiEYS CURRENT REGULATE D POWER SUPPLY United States Patent O- Int. Cl. H01s 1/00 US. Cl. 330--4 29 Claims ABSTRACT OF THE DISCLOSURE A high-gain, broadband traveling-wave maser of the folded-comb type is provided by a structure which increases gain, and structure which stagger tunes sections thereof with some sacrifice of gain in order to improve the bandwidth by a factor of about two. Gain is improved by: a unitary or integral comb and wave-guide structure; adjustable signal coupling loops; use of single crystal yttrium iron garnet isolators supported by active material of the same type and quality as the maser material; use of zero-degree Czochralski ruby as the active material; improved means for clamping active material in the comb and waveguide structure; and a comb shape design which optimizes the phase filling factor. Stagger tuning of sections of the folded traveling-wave maser is achieved by iron shims and conductive coils to selectively modify the external magnetic field for the desired band width.

ORIGIN OF THE INVENTION The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 USC 2457) BACKGROUND OF THE INVENTION This invention relates to a maser amplifier of the comb type, and more particularly to structure which provides an improved gain-bandwidth product and noisetemperature characteristic.

The three-level solid state traveling-wave maser (TWM), first described by R. W. De Grasse, et al., in The Bell System Technical Journal, vol. 38, (March 1959) at pages 305 to 334, is now well known and widely used. For greater amplification, a folded-comb structure of the type described in US. Patent 3,299,364 has been developed. However, an S-band traveling-wave maser of that construction has been found to provide a net gain of 26 db with a bandwidth of only 18 mHz. and a noise temperature of 12 K.

Extended space communications requirements call for a TWM with a low equivalent input noise-temperature characteristic and a high gain-bandwidth product. Large antennas have provided noise temperatures of less than K. at S-band, for example. Accordingly, the TWM should have a very low noise temperature, preferably below 6 K. within the desired frequency range. It should also have a high gain-bandwidth product. Current requirements are for about 45 db net gain and a 1 db bandwidth extending from 2270 to 2300 mHz.

SUMMARY OF THE INVENTION The preferred embodiment of the invention disclosed herein comprises a pair of parallel waveguide sections having a common wall, each with a unitary comb extending from a base and lying in a plane parallel to the com- CCi mon wall. Active material is placed on each side of the comb in both waveguides and pressed against the comb and the common wall by spring force. The channel between the comb and the common wall is filled with a slab of active material to approximately /3 the height of the-com=b such that the active material is in contact with the base from which the comb extends. The other channel of each waveguide section is similarly filled with a narrow slab of active material to support properly spaced isolators. The balance of the other channel is filled to approximately the same height as the one channel with a slab of active material. The two waveguide sections enter a short waveguide extension common to both. This short extension is covered at its remote end by a plate to close the assembly at the fold thereof. The two waveguide sections are coupled by an internal loop of conductive wire which passes through the waveguide extension. The other ends of the two waveguide sections are extended significantly beyond the first fingers of the combs to provide isolated input and output signal channels. Those two channels enter a common short waveguide extension which ends in a flange having a port for the mocrowave pump power. The flange is adapted to be securely fastened in close contact with a cryogenically cooled supporting plate.

The design of the comb fingers has been optimized to improve gain per unit length across a wide tuning range in a manner suggested by F. F. Chen and W. J. Taber, in the Bell System Technical Journal, vol. 43, (May 1964) at pages 1005 to 1033.

In accordance with a further feature of the present invention, the slabs of active material are cut from a zero-degree Czochralski ruby boule with their lengths along the C-axis of the boule.

In addition to the foregoing, the present invention provides as a further feature improved signal power coupling between input and output coaxial signal transmission lines and the comb-type slow wave structure, and between sections of the folded-comb structure. Each coupling consists of a loop of conductive wire grounded at the base of the comb just off the end thereof. In each instance, a substantial portion of the coupling loop lies in the plane of the adjacent comb. Adjustments for matched coupling between the input and output coaxial signals are made by simply bending the loops to vary the distance from the adjacent combs.

The bandwidth of the TWM is selected in accordance with still another feature of the invention by so modifying the DC magnetic field in various sections of the combs with shims of magnetic material and coils as to provide stagger tuning. DC current is adjusted in the coils to modify the DC magnetic field in various sections of the combs for fine adjustments in the stagger tuning arrangement. In that manner, some of the high gain provided by the foregoing features is traded for additional bandwidth. The resulting reduction in net gain causes an increase in noise temperature across the bandwidth, but not beyond the desired maximum noise temperature.

The various novel features embodied in the invention disclosed herein may also be .used to advantage in a single waveguide section extended to the length required for the amplification desired, but two shorter sections in a folded arrangement is preferred. Accordingly, although a preferred embodiment is disclosed, the novel features considered characteristic of this invention are set forth with particularity in the appended claims. The invention will best be understood from the following description of the preferred embodiment with reference to the accompanying drawings.

3 BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 on the line 44 thereof.

FIGURE 5 is an exploded view of the TWM of FIG URE 1.

FIGURE 6 is a schematic plan view of the TWM of FIGURE 1 illustrating the manner in which stagger tuning is achieved byshims of magnetic material" and 'a figure-8 coil.

DESCRIPTION OF THE PREFERRED EMBODIMENT In a preferred embodiment of the invention, a folded TWM is provided as shown in FIGURE 1. It includes a main body 10 and a cover 11 made of highly conductive non-magnetic material, preferably copper. The body 10 is machined to provide two longtudinal wave-guide sections 12 and 13 having a common or dividing wall 14. The body 10 is also machined to provide unitary combs and 16, as will be described more fully with reference to FIGURES 2 and 4.

An end plate 17 provides a waveguide short to close the waveguide sections 12 and 13. It should be noted that the dividing wall 14 and combs 15 and 16 do not extend to the end plate 17 in order to provide a short waveguide extension 18 common to both sections 15 and 16. A loop 19 of electrically conductive, non-magnetic wire is provided in that extension 18 to couple one comb to the other around the dividing wall 14 as shown in FIGURE 3. Each end of the loop 19 is grounded in the base 20 of the main body 10 as represented by the dotted line extensions of the loop 19.

A front flange 21 closes the waveguide sections 12 and 13 at the opposite end of the main body 10 from the end plate 17. The front flange 21 is adapted to be connected to a pump power source (not shown) operating at a desired frequency so that energy passes through a port 22 into the waveguide sections 12 and 13 having the slowwave structure comprising combs 15- and 16 end-coupled thereto. The microwave pump power is supplied to the port 22 in the flange by a waveguide which extends through a refrigeration system to a microwave power source at room temperature. The same flange 21 provides a sufficiently large mounting surface, for contact with a cryogenically cooled cold plate. Cryogenic cooling is accomplished by conduction and the TWM is operated in a high vacuum which provides insulation from the room temperature environment around the cryogenic refrigerator.

The dividing wall 14 extends toward, but not to the flange 21, as may be seen in FIGURE 6, in order that pump power passing through the port 22 will enter both waveguide sections 12 and 13.

The combs 15 and 16 extend toward the flange 21 along the length of the dividing wall 14 to points in the waveguide sections 12 and 13 close to where input and output coaxial cables 23 and 24 are connected. Standard coaxial connectors 25 and 26 are provided on the cables 23 and 24 for connection to a signal-source and signaldetection apparatus (not shown). The external conductor of each cable, such as conductor 23 of the cable 23 shown in FIGURE 2, is connected to the outside wall of a waveguide section. The internal conductor is then grounded by connection to the base of the unitary waveguide structure near the first resonant element (finger of the comb), as shown in FIGURE 2 for internal conductor 23". The space between the internal and external cable conductors is filled with a suitable dielectric material. Thus, coupling of the input and output cables to the slow-wave structure (combs 15 and 16) is accomplished by using loops adjacent thereto in the same manner that loop 19 couples one slow-wave structure (comb 15) to the other (comb 16).

Signal coupling adjustments are readily made for maximum power transfer by bending the coupling loops, thereby changing the distances to the adjacent resonant elements of the slow-wave structures. For instance, to adjust the input signal coupling, the conductor 23" is bent to the desired position with a substantial portion thereof in the plane of the slow-wave structure shown in longitudinal section in FIGURE 2. The loop 19 is similarly bent to a desired position with substantial portions in the planes of the slow-wave structures as shown in the end view of FIGURE 3. The opposite ends of the loop 19 are grounded to the base 20 of the main body 10 in the same manner as shown for the conductor 23" in FIGURE 2, which is by soldering into bores in the base 20. The loop 19 and the conductors of the coaxial cables 23 and 24 are made of highly conductive non-magnetic material, preferably copper.

In accordance with another even more important feature of the present invention, the resonant elements (fingers of combs 15 and 16) are machined from the same bar of material as the main body 10 to provide a unitary or integral structure as shown in the crosssectional views of FIGURES 2 and 4. In that manner, joints are eliminated in the critical areas where the resonant elements (fingers) of the slow-wave structure meet the base 20 of the main body 10. This improves the gain of the TWM because joints in the critical areaswould result in loss of microwave energy due to possible leakage and power losses in such joints.

The unitary structure of the main body 10 and combs 15 and 16 is formed by first milling a bar of the proper length and then shapiting it with precision to a form having a uniform cross-section as shown in FIGURE 4 along its entire length. The fingers of the comb are then formed with precision by electric discharge machining to the form shown in the longitudinal section of FIGURE 2. In addition, the wall 14 is shortened at each end for reasons which will become more apparent.

The cover 11 is machined from a separate bar of the same material as the main body 10 and secured in place by screws 27 along the outside walls of the main body 10 and the dividing wall 14. The cover 11 need not be secured to the main body with any better connection than is provided by the pressure of the screws 27 since virtually all of the microwave energy is being propagated by the resonators (fingers of combs 15 and 16). Although the RF electric (E) field is maximum at the ends of the fingers, the RF magnetic (H) field is at a minimum and no current flows through the cover 11.

It should be noted that suitable longitudinal slots 32 and 33 (FIGURE 5) are machined along the inside of the cover 11 to assure that the resonators are not grounded at their upper free ends where the maximum E field exists. Instead of machining the slots 32 and 33 in the cover 11, the fingers of the combs 15 and 16 could be machined shorter, but it is preferred to have the fingers of the same height as the side walls of the waveguide sections 12 and 13, and the dividing wall 14, for ease of manufacture.

The capacitance (gap) between the cover 11 and the end of at least one finger at each end of the comb is adjusted for maximum power transfer into and out of the comb structure. For example, the gap between the cover 11 and the first finger of the comb 16 is adjusted by a screw 34 and a lock nut 35. The second finger is similarly tuned by screw 36 and lock nut 35. The last and penultimate fingers of the comb 16 are also tuned in the same manner.

The center frequency of the TWM can be shifted by adjusting the height of active maser material in the waveguide sections 12 and 13 on each side of the combs 15 and 16. At 2300 mc., a height of 0.475 in. for the maser material is used with 'a finger length of 0.720 in. The center frequency shifts at a rate of 4 mo. per 0.001 in. of change in maser material heightjThe maser material should always rest on the base of the comb structure, as shown in FIGURE 4, where the maximum H field exists.

In accordance with an important feature of the present invention, gain of the TWM is greatly improved by use of O-degree ruby grown by the Czochralski process as the active material. Ruby crystal slabs are cut to fit slots beside the combs with close tolerances and with their Q axis along their length. Gain is improved because the ruby slabs cut from the Czochralski grown boule are substantially free of flaws which would degrade the effec tive filling factor (F) and gain is determined by the following equation:

Gdb=27.3 SNF/Qm Where N is the active length of the TWM in free space wavelengths, Qm is the magnetic Q of the material, and S is the slowing factor computed as the ratio of light velocity to group velocity.

Ruby slabs of the desired width and thickness are placed between the dividing wall 14 land the combs 15 and 16, as shown by slabs 41 and 42. The slabs 41 and 42 are of sufficient length to extend beyond each end of the combs 15 and 16. Similar slabs 43 and 44 are then provided on the other side of the combs 15 and 16, but of lesser width in order to provide room below for isolator strips 45 and 46.

The manner in which maser material is ntilied to support isolators will now be described with reference to FIGURES 3 and 4. The isolator strips 45 and 46 are preferably also ruby slabs cut from -a O-degree Czochralski boule in order that the waveguides 12 and 13 be filled as much as possible with active material around the base of the combs 15 and 16 to improve the gain of the TWM. The isolators supported by the strips 45 and 46, such as isolators 47 and 48, are single-crystal YIG. A comparison of the single-crystal YIG and a polycrystalline YIG shows an improved figure of merit (ratio of reverse loss to forward loss). This also contributes to the greatly improved gain of the present invention. This is because a TWM configuration which obtains highest gain has been found to degrade isolator performance in such a way as to result in high forward loss with polycrystalline YIG. The high forward loss is reduced by the improved figure of merit for single-crystal YIG. Accordingly, the single-crystal YIG allows the use of a TWM configuration that yields higher gain than could be obtained with designs using polycrystalline YIG.

The shape of the comb fingers which yields highest gain forces the major component of the RF magnetic field to lie in a direction parallel to the C axis of the ruby maser material. Accordingly, the shape of the fingers is selected to be one having a rectangle cross-section with the longer dimension parallel to that C axis which extends parallel to the line of comb fingers. The final dimensions to be used are determined empirically. For example, for S-band amplification, optimum gain was achieved with combs having 40 fingers, each finger 0.080 in. wide and 0.030 in. thick, with a space between fingers of 0.070 in. For operation at 2300 mc., the finger length selected was 0.720 with a slab height of 0.475, as noted hereinbefore.

The resonant frequency of the isolator material is determined by the shape of the YIG and the applied magnetic field. For maser operation, the magnetic field required is determined by the active material employed. The YIG isolator shape is then adjusted for resonance at the proper frequency, normally by lapping the thickness of a YIG disc. In a preferred embodiment successfully tested, the isolator disc was selected to have a diameter of 0.045 in. The thickness was then adjusted for isolation over a frequency range of 2220 to 2350 mHz.

It is to be understood that an isolator disc is positioned opposite each space between fingers of a comb, although in practice one isolator is omitted at about the center of the comb structure and a thicker disc of passive material, such as alumina, substituted to space the crystal slab slightly above it (to avoid damaging the isolator discs when the slabs 43 and 44 are installed). Thus, with sufliciently long crystals 43 and 441 to extend the full length of the combs 15 and 16, only three alumina spacers are required in each waveguide section, one at the center and one at each end, such as spacers 51 and 52 seen at the end shown in FIGURE 3.

The precision-fit slabs 41 to 46 are clamped by an improved spring assembly comprising a beryllium copper strips 55 on each side of the main body 10 secured thereto by a plurality of screws (such as screw 56) evenly spaced along a straight line, as shown in FIGURE 1. Offset from each screw, and alternately above and below the line of screws, brass pins are inserted in bores through the side walls of the main body 10, such as the pin 57 shown in the cross-sectional view of FIGURE 4. Each such pin is longer than the side wall is thick. When the screws 56 are tightened, each strip 55 functions as a plurality of leaf springs exerting force against pins to clamp the slabs 43 and 44 against the combs and toward the dividing Wall 14. Thus, pressure is transferred through the combs 15 and 16 to slabs 41 and 42 which in turn are pressed against the center wall 14. Similar pins, but of smaller diameter, are inserted into bores through the side walls of the main body 10, such as a pin 58, through which the strips 55 clamp the isolator slabs 45 and 46 against the base of the combs 15 and 16. In that manner all slabs of active material 41 to 46 are so clamped as to provide maximum area contact with the combs 15 and 16 and with the dividing wall 15 for maximum heat transfer through the main body 10 to the flange 21.

An S-band TWM of the folded-comb type was successfully constructed with base features, and tested. It was found to have such significantly improved gain that, upon stagger tuning for a bandwidth of 46 MHz, it still had a net gain of 46 db with a noise temperature of about 6 K. That compared to a net gain of about 35 db for an S-band TWM of the best previously available which had a bandwidth of 18 mHz. and a noise temperature of about 12 K. The assembly of the component parts of such a greatly improved TWM may be better understood from the exploded view of FIGURE 5. In that view, the DC magnetic field (H is indicated schematically as being parallel to the fingers of the combs 15 and 16. This field of about 2500 gauss is supplied by a pound alnico magnet (not shown).

To provide efficient stagger tuning (one which trades excess gain for additional bandwidth efliciently), shims of magnetic material are employed along the length of the folded-comb TWM of any length required to achieve the bandwidth desired, such as a shim 61, 62 and 63 shown in FIGURE 5. They are secured to the bottom of the TWM by suitable non-magnetic screws. In addition to the shims, which alter the permanent magnetic field (H conductive wire is so wound around forms 65 and 66 as to form a FIGURE 8, or two coils with oppositely wound turns. DC current is then adjusted to further provide magnetic field staggering for broad bandwidth tuning of sections of the TWM. This technique provides a more eificient gain versus bandwidth trade-off than the use of a single coil as in the prior art.

It should be noted that the forms 65 and 66 are made of non-magnetic material as are all other elements (pins, screws, etc.). Only the shims are of magnetic material. Since they are purposely made of different thicknesses, and it is desirable to provide an even surface for the coil forms 65 and 66, non-magnetic shims (not shown) may be used to even the surface presented. to the forms 65 and 66.

FIGURE 6 shows in a plan view how the magnetic field (H is modified to provide four sections of amplification that operate alternately at four different frequencies F through F The shims 61, 62 and 63 of different thicknesses are disposed along the length of the TWM with the thinest 63 on the input-output end and the thickest 61 on the opposite end. That provides three degrees of magnetic field concentration that may be referred to as light, medium and heavy. Current is then adjusted through the figure-8 coil 67 with the polarities shown to further decrease the density of magnetic flux in the area of the shim 63 and increase the fiux in the area of the shim 61. The area of the shim 62 is covered by both parts of the figure-8 coil; therefore, the right half has its flux density decreased while the left half has its flux density increased. The result is: a low flux density in the area of shim 63; medium-low flux density in the area of the right half of shim 62; medium-high density in the area of the left half of shim 62; and high density in the area of shim 61. Each field change slightly shifts the resonance frequency of the combs and 16 and results in stagger tuning of the TWM with eight sections of amplification operating at four different frequencies. As the current through the figure-8 coil 67 is increased by means of a current regulated power supply 68, the separation between the four areas (low, medium-low, medium-high, and high) is increased, thereby increasing the separation of the four frequencies and broadening the band width until the four frequencies are so separated that they begin to appear as separate peaks in the frequency response curve.

Although three shims have been illustrated in FIGURE 6, it should be understood that any number of shims may be employed to provide for various numbers of tuning sections. Additional coils may also be employed to adjust the separate sections.

What is claimed is:

1. In a traveling wave maser of the comb type, the combination comprising:

a waveguide section having parallel sidewalls and a comb structure therebetween, said side walls and comb being machined from a bar of electrically conductive, non-magnetic material with a common base;

a first slab of active maser material precision fit in said waveguide section between one of said walls and said comb with one edge thereof substantially in contact with said base;

a second slab of active maser material ruby precision fit in said waveguide section between the other one of said walls and said comb with one edge thereof substantially in contact with said base, said second slab extending up the side of the comb to a height substantially less than said first slab; plurality of isolators disposed along the upper edge of said second slab, each isolator being positioned opposite a space between fingers of said comb; and

a third slab of maser material precision fit in said waveguide section between the other one of said walls and said comb with one edge thereof spaced slightly away from said isolators, said third slab extending up the side of said comb to a height substantially equal to the height of said first slab.

2. The combination as defined in claim 1 wherein said active maser material for said first, second and third slabs is O-degree Czochralski grown ruby.

3. The combination as defined in claim 2 wherein said slabs are cut with the length thereof along the C axis of said ruby.

4. The combination a defined in claim 1 wherein each of said isolators is made of singlecrystal yttrium iron garnet.

5. The combination as defined in claim 4 wherein said comb is designed for optimum gain with fingers having a substantially rectangular cross-section with the long dimension thereof parallel to the line of the comb fingers.

6. The combination a defined in claim 5 wherein said active maser material for said first, second and third slabs is O-degree Czochralski grown ruby, and each of said slabs is cut with its long dimension parallel to the C axis of said ruby.

7. The combination as defined in claim 6 including means for pressing said first, second and third slabs toward said one of said walls, whereby maximum contact with said one of said walls for said comb is provided through said first slab, and maximum contact of said comb with said first, second and third slabs is assured.

8. The combination as defined in claim 7 wherein said clam ing mean comprises a strip of spring material secured to the outside of the other one of said walls and pins passing through ports in said other one of said walls beneath said strip, said pins being longer than said other one of said walls is thick, whereby pressure from said spring strip is transferred to said second and third slabs toward said comb and said first slab by said pins.

9. The combination as defined in claim 8 wherein:

at least one shim of magnetic material disposed near said comb along a section thereof in a plane perpendicular to fingers of said comb, whereby magnetic flux concentration is varied over said section of said comb;

a plurality of coils disposed near said comb along sections thereof in a plane perpendicular to fingers of said comb; and

mean for adjusting current amplitude through said coils, whereby magnetic flux density may be adjusted by sections to provide, in cooperation with said shims, stagger tuning of sections of said traveling wave maser.

10. In a high-grain traveling wave maser of the type having a comb in a waveguide section, the combination comprising:

at least one shim of magnetic material disposed near said comb along a section thereof in a plane perpendicular to fingers of said comb, whereby magnetic flux concentration is varied over said section of said comb;

a plurality of coils disposed near said comb along sections thereof in a plane perpendicular to fingers of said comb; and

means for adjusting current amplitude through said coils, whereby magnetic flux density may be adjusted by sections to provide, in cooperation with said shims, stagger tuning of sections of said traveling wave maser.

11. The combination of claim 10 wherein said high gain traveling wave maser is achieved by structure comprising:

a waveguide section having parallel sidewalls and a comb structure therebetween, said side walls and comb being machined from a bar of electrically conductive, non-magnetic material with a common base;

a first slab of active maser material precision fit in said waveguide section between one of said walls and said comb with one edge thereof substantially in contact with said base;

a second slab of active maser material ruby precision fit in said waveguide section between the other one of said Walls and said comb with one edge thereof substantially in contact with said base, said second slab extending up the side of the comb to a height substantially less than said first slab;

a plurality of isolators disposed along the upper edge of said second slab, each isolator being positioned opposite a space between fingers of said comb; and

a third slab of maser material precision fit in said waveguide section between the other one of said wall and said comb with one edge thereof spaced slightly away from said isolators, said third slab extending up the side of said comb to a height substantially equal to the height of said first slab.

12. The combination of claim 11 wherein said active maser material for said first, second and third slabs is O-degree Czochralski grown ruby.

13. The combination of claim 12 wherein said slabs are cut the length thereof along the C axis of said ruby.

14. The combination of claim 11 wherein each of said isolators is made of single-crystal yttrium iron garnet.

15. The combination of claim 14 wherein said comb is designed for optimum gain With fingers having a substantially rectangular cross-section with the long dimension thereof parallel to the line of the comb fingers.

16. The combination of claim 15 wherein said active maser material for said first, second and third slabs is O-degree Czochralski grown ruby, and each of said slabs is cut with its long dimension parallel to the C axis of said ruby.

17 The combination of claim 16 including means for pressing said first, second and third slabs toward said one of said walls, whereby maximum contact with said one of said walls for said comb is provided through said slab, and maximum contact of said comb with said first, second and third slabs is assured.

18. The combination of claim 17 wherein said clamping means comprises a strip of spring material secured to the outside of the other one of said walls and pins passing through ports in said other one of said walls beneath said strip, said pins being longer than said other one of said walls is thick, whereby pressure from said spring strip is transferred to said second and third slabs toward said comb and said first slab by said pins.

19. In a traveling wave maser having two waveguide sections, each section having parallel walls and a comb structure therebetween, said sidewalls and comb being machined from a bar of electrically conductive, nonmagnetic material with a common base between said side walls, one of said side walls being common to both sections and the other one of said side walls being on the outside, and said base being shared by both sections, the combination for each waveguide section comprising:

a waveguide section having parallel sidewalls and a comb structure therebetween, said side walls and comb being machined from a bar of electrically conductive, non-magnetic material with a common base;

a first slab of active maser material precision fit in said waveguide section between one of said walls and said comb with one edge thereof substantially in contact with said base;

a second slab of active maser material ruby precision fit in said waveguide section between the other one of said walls and said comb with one edge thereof substantially in contact with said base, said second slab extending up the side of the comb to a height substantially less than said first slab;

a plurality of isolators disposed along the upper edge of said second slab, each isolator being positioned opposite a space between fingers of said comb; and

a third slab of maser material precision fit in said waveguide section between the other one of said walls and said comb with one edge thereof spaced slightly away from said isolators, said third slab extending up the side of said comb to a height substantially equal to the height of said first slab.

20. The combination as defined in claim 19 including a short waveguide extension common to both of said sections and a loop of electrically conductive, non-magnetic wire disposed in said extension to couple one section to the other, one end of said loop being grounded near the comb of one waveguide section at the base thereof and in line with fingers of the comb, the other end of said loop being grounded near the comb of the other waveguide section at the base thereof and in line with fingers of the com-b, a substantial portion of said loop being substantially parallel to the end finger of the comb in one section and another substantial portion being substantially parallel to the end finger of the comb in the other section, each of said portions being adjustable in position from its proximate comb for maximum power transfer, whereby one waveguide section is efficiently coupled to the other around said one common wall and through said loop in said waveguide extension.

21. The combination as defined in claim 20 including input and output coaxial cables, each cable having its outer non-magnetic conductor grounded through a side wall to said base and having its inner non-magnetic conductor passing through said side wall into a different one of said two waveguide sections where the end thereof is grounded near the comb therein at the base thereof at the end opposite to said loop and in line with fingers of the comb, a substantial portion of said inner conductor being substantially parallel to the finger of the comb at the end opposite said loop, said portion of said conductor being adjustable in position from its proximate comb for maximum power transfer between the coaxial cable of the inner conductor and the waveguide section of the proximate comb.

22. The combination as defined in claim 21 wherein said active maser material for said first, second and third slabs is O-degree Czochralski grown ruby.

23. The combination as defined in claim 22 wherein said slabs are cut with the length thereof along the C axis of said ruby.

24. The combination as defined in claim 21 wherein each of said isolators is made of single-crystal yttrium iron garnet.

25. The combination as defined in claim 24 wherein said comb is designed for optimum gain with fingers having a substantially rectangular cross-section with the long dimension thereof parallel to the line of the comb fingers.

26. The combination as defined in claim 25 wherein said active maser material for said first, second and third slabs is O-degree Czochralski grown ruby, and each of said slabs is cut with its long dimension parallel to the C axis of said ruby.

2.7. The combination as defined in claim 26 including means for pressing said first, second and third slabs toward said one of said walls, whereby maximum contact with said one of said walls for said comb is provided through said first slab, and maximum contact of said comb with said first, second and third slabs is assured.

28. The combination as defined in claim 27 wherein said clamping means comprises a stri of spring material secured to the outside of the other one of said walls and pins passing through ports in said other one of said walls beneath said strip, said pins being longer than said other one of said walls is thick, whereby pressure from said spring strip is transferred to said second and third slabs toward said comb and said first slab by said pins.

29. The combination as defined in claim 28 wherein:

at least one shim of magnetic material disposed near said comb along a section thereof in a plane perpendicular to fingers of said comb, whereby magnetic flux concentration is varied over said section of said comb;

a plurality of coils disposed near said comb along SEC-x tions thereof in a plane perpendicular to fingers of said comb; and

means for adjusting current amplitude through said coils, whereby magnetic flux density may be adjusted by sections to provide, in cooperation with said shims, stagger tuning of sections of said traveling wave maser.

References Cited UNITED STATES PATENTS 3,299,364 1/1967 Buchmiller et all. 3304 RODNEY D. BENNETT, JR., Primary Examiner CHARLES E. WANDS, Assistant Examiner 

